The present invention generally relates to electrically driven compressors and to the cooling of electrical motor/generators and, more particularly, to a system and method for optimized thermal and secondary flow management of electrically driven compressors.
Commercial jet aircraft fly at very high altitudes, and therefore, their cabin air systems must provide a safe, comfortable, and pressurized environment. Modern aircraft are often equipped with an environmental control system (ECS) where the fresh air for the cabin is provided by electrical motor-driven compressors in place of traditional engine bleed air. Cabin air compressors have a wide operating envelop. In order to have a reliable and long lasting system, it is essential to have a thermally stable components; this applies in particular to the electrical stator and rotor as well as the bearings. Generally, a considerable amount of heat is generated during the operation of an electrical motor or generator, and cooling the space between the rotor shaft and the stator, the rotor shaft and bearings, as well as the housing and stator, especially when the motor is operated at high speeds, is required. Frictional heating occurs as the rotor spins at high speed, but heating also occurs as electric current flows through the rotor and stator coils as they rotate relative to one another in the magnetic fields.
Therefore, electrically driven motors and generators are generally equipped with cooling systems, such as gas ventilation systems, to transfer heat from the stator and rotor. A gas ventilation system cools the rotor and stator by forcing cooling gas through gas passages in the rotor and stator. U.S. Pat. No. 2,692,956 and U.S. Pat. No. 2,787,720, for example, cool the rotor and stator with air.
It is further known in the art that electrical motors/generators may have one cooling system for the rotor element and another cooling system for the stator element. The two cooling systems are typically maintained separately from one another and each can be in a closed recirculation path. U.S. Pat. No. 3,089,969, for example, provides a cooling system for turbo-generators where the rotor space is sealed in a gas tight manner from the stator space and wherein the rotor is cooled by circulating cooling gas through the rotor element while the stator is cooled by circulating a cooling liquid, such as oil, through the stator element. The closed space occupied by the stator element is further filled with an incombustible gas, such as carbon dioxide or air, that is separated from the cooling liquid.
In another example, the space occupied by the rotor is sealed off from the space occupied by the surrounding stator element, which makes it possible to utilize a nonconductive liquid, such as oil, for cooling of the stator element and the winding placed therein. U.S. Pat. No. 5,271,248, for example, uses an oil loop to extract heat from the stator by conducting the heat from the stator core and winding to the oil. A separate refrigerant vapor loop is used to cool only low temperature electronics and not the electrical motor/generator.
While cooling a rotor with a cooling gas, such as air, may be effective for cooling a rotor, it may not be effective enough to cool a stator of a high power density electrical motor/generator to the desired operating temperatures. Furthermore, while wet-cooling a stator with a nonconductive cooling liquid, such as oil, may be effective for extracting the operational heat from the stator, the space surrounding the stator must be sealed off, for example, by using a bore-seal between the stator and rotor. This may increase the manufacturing cost of the electrical motor/generator as well as lowering the overall availability of the system due to inherent breakdown of sealing. Wet-liquid cooling the stator may also mean that a larger gap between the rotor and the stator is needed, which may result in a lower overall motor/generator efficiency and higher losses. An alternative to wet-liquid cooling the stator may be dry-liquid cooling the stator by using a cooling jacket that surrounds the iron stack and winding of the stator. In this case the end turns of the stator winding may need to be potted for cooling. Potting of end turns is not reliable since pieces can separate from the potting material and may fall into the rotating group, which can cause motor/generator seizure.
As can be seen, there is a need for a system and method for optimized thermal and secondary flow management of electrically driven compressors. Furthermore, there is a need for separation and optimal distribution of the thermal loads that occur during the operation of an electrical motor/generator, as well as for optimization of the cooling of rotor and stator elements to increase the overall cooling effectiveness of electrically driven compressors.